The amino acid L-glutamate is a major excitatory neurotransmitter in the mammalian central nervous system. Anatomical, biochemical and electrophysiological analyses suggest that glutamatergic systems are involved in a broad array of neuronal processes, including fast excitatory synaptic transmission, regulation of neurotransmitter releases, long-term potentiation, learning and memory, developmental synaptic plasticity, hypoxic-ischemic damage and neuronal cell death, epileptiform seizures, as well as the pathogenesis of several neurodegenerative disorders. See generally, Monaghan et al., Ann. Rev. Pharmacol. Toxicol. 29:365-402 (1980). This extensive repertoire of functions, especially those related to learning, neurotoxicity and neuropathology, has stimulated recent attempts to describe and define the mechanisms through which glutamate exerts its effects.
Currently, glutamate receptor classification schemes are based on pharmacological criteria which serve to define five receptor subtypes or classes: those activated by N-methyl-D-aspartic acid (NMDA), kainic acid (KA), .alpha.-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA, formally called the quisqualic acid or QUIS receptor), 2-amino-4-phosphonobutyric acid (AP4 or APB), and 1-amino-cyclopentyl-1,3-dicarboxylic acid (ACPD). The effects of glutamate are mediated primarily through interactions with cation-selective, ionotropic receptors [Foster and Fagg, Brain Res. 7:103-164 (1984); Strange, Biochem. J. 249:309-318 (1988)]. An exception is the ACPD receptor subtype which has the properties of a metabotropic receptor. This class of glutamate receptors alters synaptic physiology via GTP-binding proteins and the second messengers diacylglycerol and inositol 1,4,5-triphosphate [Gundersen et al., Proc. R. Soc. London Ser. 221:127 (1984); Sladeczek et al., Nature 317:717 (1985); Nicoletti et al., J. Neurosci. 6:1905 (1986); Sugiyama et al., Nature 325:531 (1987)].
The electrophysiological and pharmacological properties of the glutamate receptors have been extensively studied and are now well established. See, for example, Foster and Fagg, Brain Res. Rev. 7:103 (1984); Cotman et al., Trends Neurosci. 10:263 (1987); Mayer and Westbrook, Prog. Neurobiol. 28:197 (1987); Watkins and Olvermann, Trends Neurosci. 10:265 (1987): and Blair et al., Science 242:577 (1988). This is in contrast to their biochemical characteristics and structure at the molecular level, which, until the teaching of the present invention, remained largely unknown.